1. Field of the Invention
The present invention relates in general to the field of integrated circuits, and more specifically to a system and method for minimizing end boundary resistance in programmable resistors of an integrated circuit.
2. Description of the Related Art
Integrated circuits use programmable resistors to fine tune resistances for semiconductor devices. Typically, designers utilize computer programs to simulate integrated circuit functionality prior to fabrication. Sometimes integrated circuit fabrication produces functional variations that do not match simulations. Programmable resistors are used to offset the fabrication variations and conform actual fabricated integrated circuits with design simulations. To achieve a high degree of conformance, designers attempt to use programmable resistors with minimum resistance increments. For example, setting and calibrating a bias point can require programmable resistor increments on the order of 100 ohms.
FIG. 1 depicts a top view of a conventional programmable resistor, fabricated in a semiconductor substrate and sometimes referred to as a “meander resistor”. The meander resistor 100 includes 6 taps, A through F, connected to respective metal layers 101A–101F. Salicide contacts 102a–102j connect metal layers 101A–101F to respective primary resistive areas 104AB, 104BC, 104CD, 104DE, and 104EF extending between each tap and having respective length and width dimensions of “L” and “w”. Resistive areas 104AB–104EF are, for example, a p+ diffusion area or non-salicide polysilicon resistor. A salicide deposition blocking layer 104 preserves the relatively high resistance (compared to a salicide) of resistive areas 104AB–104EF. “Rsq·(L/w)” defines the resistance of resistive areas 104AB–104EF, where “Rsq” represents the unit area resistance. An inherent parasitic resistance Rend/w exists at each end boundary region between a salicide contact 102X and a resistive area 104Y, where “X” represents a corresponding contact to a resistive area “Y” and “Rend” represents a unit area resistance. A1, B1, B2, . . . , F1 identify end boundary regions. The value of resistance Rend is inversely proportional to the width, w, of the end boundary region.
The meander resistor 100 is programmed by selecting the beginning and end taps through which current will flow. For example, by selecting taps A and C, current will flow from tap A, across metal 101A, contact 102a, end boundary A1, resistive area 104AB, end boundary B1, contact 102b, metal 101B, contact 102c, end boundary B2, resistive area 104BC, end boundary C1, contact 102d, metal 101C, to tap C. For many semiconductor structures, the end boundary resistance Rend/w between each contact 102X and resistive area 104Y is approximately equal to Rsq·(L/w). Thus, the parasitic resistance Rend/w in a meander resistor 100 contributes approximately twice the amount of resistance of the resistive areas 104Y for each tap combination. For example, programming a resistance configuration of RAF (i.e. selecting taps A and F) results in an approximate resistance RAF=5Rsq·(L/w)+10Rend/w. The large contribution of Rend/w to the overall resistance of a meander resistor 100 disadvantageously limits the granularity of achievable resistance values. For example, if Rsq·(L/w)=Rend/w=100 ohms, the granularity of meander resistor 100 is limited to increments of 300 ohms. This level of granularity is often not ideal when fine tuning semiconductor devices such as voltage controlled oscillators.